When you double-click a photo in iPhoto to look at it full screen, it makes a copy of it on the chance that you intend to edit it. It stores that in a shadow directory called "Modified". Even if you don't edit it, the copy is retained.

Right click on iPhoto Library, Show Package Contents, look at size of Modified vs Originals folders to see if this is the cause behind your library doubling. If it is the case, please report back, I'd like to know as I also do not care for the implementation of this functionality.

If you crop and such your photos then you can't just delete the Modified ones without trouble.